This invention relates to roll-damping tanks.
Open topped water tanks running across ships have been used for approximately 100 years for the damping out of rolling motions. These fundamentally consist of a rectangular tank running athwartships, spaced above the roll centre if possible, and containing water to approximately one-third of their depth, and such tanks damp out roll by the water in them oscillating to and fro in antiphase to the movement of the ship. The velocity of the water in the tank across the ship is determined by its depth and hence for a given period of roll of the ship the period of oscillation in the water in the tank can be adjusted to match. This has been well known for a hundred years and is a long-established state of the art.
The introduction of constrictions in the tank is common. Indeed restrictions were introduced in quite early tanks, the idea being that the energy absorbed by such restrictions as the water flowed over or round them in a turbulent manner would increase the damping of the ship. This is a fallacy. Any restrictions tend to restrict the roll damping moment available compared with the free transfer of water across the ship, and from this narrow point of view they are disadvantageous.
However, it is essential that the phase angle of the roll damping moment be correct in relation to the roll of the ship and it is found in some cases, especially for example with ships with long rolling periods, that the correct phase relationship is very difficult to obtain with a simple tank without constrictions because there is not enough damping in the tank to prevent secondary oscillations of the water induced by the efffect of the ship on the tank water. Hence the major purpose for introducing constrictions in tanks is to adjust the phase angle, and the secondary purpose is to prevent excitation of the tank by the ship in such a manner at very low and very high frequencies that the roll is actually increased. By destroying the velocity of the wave in the tank this phase angle can be modified and the product of stabilising moment and phase angle can be increased even though the stabilising moment itself is decreased.
The present invention relates to means of effecting this by choice of the shape of the tank rather than by way of actual restriction to flow.
In plan view such a tank is normally rectangular, and narrower in the fore and aft direction of the ship than in the transverse direction, in which latter direction the tank generally, if possible, extends the full breadth of the ship. As already stated, the water is usually approximately one-third the depth of the tank.